This invention concerns a device for reducing the fraction of volatile organic compounds (VOC) being separated from oil, especially crude oil, during filling of larger tanks, as is customary when, for example, loading a ship from a terminal, a production platform or a floating loading device.
Crude oil normally contains fractions of lighter and heavier gasses that, due to their volatility, may not be transported together with the liquid fraction of the crude oil. From a transport technical point of view, it would be advantageous if the crude oil was separated into a gas fraction, a wet gas fraction and a liquid fraction. To separate and transport petroleum products from a producing field in three fractions, however, is associated with significant extra costs, and therefore it is more common to separate the crude oil into a gas- and a liquid fraction. The wet gas phase therefore is divided into a lighter fraction, which is transported together with the gas fraction, and a heavier fraction, which is transported together with the liquid fraction.
During loading at a moderate overpressure of a liquid petroleum fraction containing heavier gasses, such as propane and butane, gasses from the volatile fraction continuously evaporate. In order to avoid a pressure increase in the transport and storage tanks, the evaporation gasses, i.e. the volatile organic compounds, must be extracted from the tanks and be carried onwards to incineration or to a re-injection facility.
A relatively substantial evaporation of volatile organic compounds is known to take place while pumping oil into larger tanks. A pressure in the range of 1.05 to 1.07 bar normally is maintained in both storage and transport tanks. When loading a tanker, for example, the oil normally is pumped from a storage tank and through a supply pipe to a position above the cargo tank, from which position the oil is guided into the tank through a down pipe down to the bottom portion of the tank. A down pipe of this type may be of a length in the order of several tens of meters.
When the oil flows into the upper end portion of the down pipe, the force of gravity will accelerate the liquid flowing downward in the down pipe, whereby a lesser total pressure is formed in the supply pipe and in the upper portion of the down pipe. In these pipes having a lesser total pressure, a substantial evaporation of volatile organic compounds takes place, these only being insubstantially condensed into liquid phase when the pressure again increases to the ordinary pressure of the tank.
NO 19996471 concerns a down pipe provided with a venturi, in which the venturi being arranged to condense gasses that already have separated from a cargo tank. The device according to NO 19996471 is not arranged to restrain volatile gasses from separating from the oil when being loaded into the cargo tank.